In radio communications, demodulators in radio receivers convert signals received at radio frequencies into baseband signals and decode the baseband signals to recover the original data. FIG. 1 illustrates a demodulator 100 that demodulates and decodes received radio signal 102.
In FIG. 1, demodulator 100 receives radio signal 102, which is a modulated signal that conveys original data at a radio frequency, or carrier frequency. Radio signal 102 may be analog or digital. If radio signal 102 is analog, it is converted to digital by an analog-to-digital converter (ADC), which is not specifically shown. Radio signal 102 may be a complex-valued signal. In demodulator 100, radio signal 102 is a complex-valued signal and is applied to two parallel multipliers 124 and 126. In multiplier 124, radio signal 102 is multiplied with a function cos(ωt), where ω is 2π times the carrier frequency. In multiplier 126, radio signal 102 is multiplied with a function sin(ωt), where ω is 2π times the carrier frequency. Outputs 104 and 106 are applied to low-pass filters 116 and 118, respectively, which provide anti-aliasing and remove out-of-band noise. The outputs 108 and 110 of low-pass filters 116 and 118 are the real and imaginary components, respectively, of a complex baseband signal derived from the received radio signal 102.
The outputs 108 and 110 are then each applied to decoders 120 and 122, respectively, to produce decoded signals 112 and 114, respectively. Decoders 120 and 122 receive outputs 108 and 110 and output decoded signals 112 and 114, respectively, based on the signal levels in outputs 108 and 110. Since decoders 120 and 122 perform the same functions, the discussion from this point forward will focus on a single decoder (e.g., decoder 120). Techniques discussed with respect to decoder 120 are, however, equally applicable to decoder 122.
Decoder 120 samples output 108 and generates, based on the signal level of output 108, a decoded signal 112 whose signal level comprises particular values. In an example, output 108 is a bi-level signal whose signal level is expected to be either +0.5 or −0.5 in any particular sample period. The signal level of output 108 may be expected to be either +0.5 or −0.5 in any particular sample period because it may be known that radio signal 102 is a radio signal that is based on an original baseband signal that was encoded to be either +0.5 or −0.5 in any particular sample period. In this example, decoder 120 compares the signal level of output 108 during a particular sample period to a decision value, which is 0 in this case because 0 is halfway between the encoded values of +0.5 or −0.5. If the signal level of output 108 is greater than 0 during a particular sample period, then decoder 120 will output a decoded signal 112 whose signal level is a first value. If the signal level of output 108 is less than 0 during a particular sample period, then decoder 120 will output a decoded signal 112 whose signal is a second value. The first value and second value may be +0.5 and −0.5, or any other two distinct values.
However, output 108 may also include DC offset noise, which is a low-frequency, slow-changing noise that results in output 108 exhibiting a DC offset. Waveform 202 in FIG. 2 represents a signal that is unaffected by any low-frequency, slow-changing noise and has a signal level of +0.5 or −0.5 in any particular sample period. Waveform 204 represents a low-frequency and slow-changing noise. When the noise represented by waveform 204 is added to the signal represented by waveform 202, the resultant signal, represented by waveform 206, exhibits a downward slope such that a signal level that is positive in the original signal represented by waveform 202, in a particular sample period, may be negative in that same sample period. Consequently, when the signal represented by waveform 206 is input into a decoder such as decoder 120, a decoding error will result in the particular sample period.
Various methods have been developed to remove this DC offset noise from signals so as to reduce or eliminate decoding errors. These methods include employing a low-frequency high-pass filter to remove the low-frequency components from the signals. However, these methods suffer from slow tracking bandwidth. Alternatively, a wide-band high-frequency filter may be used, but this can cause inter symbol interference. Therefore, there is a need for a method for removing DC offset noise from a signal that allows for fast tracking without decreasing the signal-to-noise ratio of the signal.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.